Signal transmission system for a measurement device

ABSTRACT

A transmission system for a measurement device on a coordinate positioning apparatus comprises a station ( 18 ) mounted on the measuring device ( 10 ) and a station ( 20 ) mounted on the coordinate positioning apparatus ( 22 ), wherein the stations communicate with each other using a spread spectrum radio link, for example frequency hopping. The station on the probe sends a regular signal and on receiving the signal the station on the coordinate positioning apparatus synchronises its clock and sends an acknowledgement signal. Measurement data is either sent in the regular signal or in a measurement event driven signal.

This invention relates to measurement device for use on coordinatemeasuring apparatus, such as a coordinate measuring machines (CMMs)machine tools, manual co-ordinate measuring arms and inspection robots.More particularly, it relates to signal transmission systems for suchmeasurement devices.

A trigger probe for determining the position of a workpiece is describedin U.S. Pat. No. 4,153,998. In use, the probe is moved by the machinerelative to a workpiece. The probe has a deflectable stylus and deliversa trigger signal when the stylus contacts the workpiece. The triggersignal is indicated by the probe switching from one state to another.The trigger signal is used by the machine controller to freeze theoutputs of scales or other measuring means which indicate the positionof the probe. The position of the point of contact on the workpiecesurface can thus be determined.

Especially on machine tools, it can be difficult to wire the probedirectly to the machine controller, and so various wireless signaltransmission systems have been developed in the prior art. These includeinductive systems (where the signal is transmitted by electromagneticinduction between two coils), optical systems (where an optical emittersuch as an infra-red diode is provided on the probe and produces anoptical signal which is received by an appropriate receiver) and radiosystems (having a radio transmitter in the probe and a radio receiverfixed at a convenient location on the machine). An example of a radiosystem is shown in U.S. Pat. No. 4,119,871. An important requirement ofsuch probes is repeatability, i.e. that the same result should beachieved every time a given measurement is repeated. The mechanicalposition of the stylus in the probes described in U.S. Pat. No.4,153,998 is extremely repeatable in space, an instant of production ofthe trigger signal always has a definite, repeatable relationship withthe instant of contact between the stylus and the workpiece. This meansthat accurate results can be obtained from the probe by a simplecalibration procedure.

However, the accuracy would be destroyed if the signal transmissionsystem were not repeatable, that is, if there were an unknown, variabledelay in the signal transmission. If this occurs then the probe wouldtravel an unknown variable distance after the instant of generation ofthe trigger signal before the machine control is able to freeze theoutputs of the measuring means. There is then an error between theposition of contact and the position indicated by the frozen outputs andthis error is an unknown variable quantity which cannot be removed bycalibration. Thus, in order to maintain overall accuracy of the probesystem there is the problem of ensuring that any transmissions delaysintroduced by the signal transmission system are repeatable i.e. thesame delay should be introduced every time the probe is triggered. Theprobe calibration procedure mentioned above will then also remove thisrepeatable delay caused by the transmission system.

U.S. Pat. No. 5,279,042 discloses an analogue radio signal transmissionsystem for a probe in which the probe is provided with a transmitter forproducing a carrier signal onto which a probe signal may be modulated. Areceiver receives the probe data and produces a probe output signalderived from the transmitter data. A clock on the transmitter provides atime standard for the whole system, the receiver uses an oscillator witha phase comparator at its input to ensure that the oscillator ispermanently synchronised with a clock in the transmitter. When a probesignal occurs, the time elapsed between the start of a counter cycle andthe change of state of the probe is latched in a shift register andtransmitted serially.

This method has the disadvantage that as the transmitter transmits acontinuous signal which is required to synchronise the transmitter andreceiver the system uses a significant proportion of the probe batterypower and thus reduces the battery life.

Furthermore, in a fixed frequency system, the number of availablecommunication channels is equal to the limited number of frequencychannels. There is therefore the problem of receivers from other systemswhich use this frequency channel intercepting the transmission sent fromthe probe. In addition, the presence of radio traffic may affect thetransmissions.

The present invention provides a transmission system for a measurementdevice for a coordinate positioning apparatus, comprising:

-   -   a first station for mounting with one of the measuring device        and the coordinate positioning apparatus;    -   a second station for mounting with the other of the measuring        device and the coordinate positioning apparatus;    -   Wherein the first and second stations communicate using a spread        spectrum radio link.

The spread spectrum radio link has the advantage of reducing the chanceof unwanted receivers intercepting a transmission, and increasing thechance of a transmission getting through to the correct receiver in thepresence of unwanted radio traffic.

The first and second stations communicate with one another using aspread spectrum radio link. This is a technique which takes a narrowband signal and spreads it over a broader portion of the radio frequencyband. Two types of spread spectrum radio link comprise frequency hoppingand direct sequencing. In frequency hopping the signal is spread byhopping the narrow band signal as a function of time. In directsequence, the signal is spread by mixing it with a special code.

The use of a periodic signal rather than a continuous signal increasesbattery life.

The measurement device may comprise a measurement probe, for example atouch trigger probe.

Preferably the transmission system uses a world-wide frequency band.

The first and second stations may be provided with a clock, wherein theclocks are synchronised at least once. The first station may transmit aregular transmission and wherein when the second station receives thesignal it may synchronise its clock with the first station. If a signaltransmitted by the first station is not adequately received by thesecond station, the signal may be retransmitted by the first station.

If the second station receives the signal transmitted by the firststation, it may transmit an acknowledgement signal. If the first stationdoes not receive an acknowledgement signal in response to its signal, itwill re-transmit said signal. The ability to re-transmit messages whichhave not been received enables the system to be capable of operating ina noisy environment.

Preferably the transmission system comprises a half duplex link.

In the event of a measurement event, the first station may transmitinformation relating to said measurement event. The measurement eventmay comprise a touch trigger event. The information may include datarelating to the time of the measurement event. The first station maytransmit a regular transmission and information relating to themeasurement event may be transmitted in an additional transmission.

The signal transmitted by the first station contains informationrelating to the measurement device, the measurement device output signalin the second station may be produced after a time delay. This timedelay is chosen so that it is long enough to allow retransmissions ofthe signal within the time delay.

A master clock is provided at one end of the transmission system and asliding correlator is provided to recover the master clock. Thisprovides a reference for the measurement device) output signal timedelay (e.g. a probe trigger output time delay). If the second stationreceives a signal from the first station it transmits an acknowledgementsignal, the acknowledgement signal sent to the first station issynchronised with the master clock. This removes the need for clockrecovery at the first station.

In a preferred embodiment, signals sent between the first and secondstations comprise data bits and data bits relating to more importantinformation are provided with greater error protection than other databits. The data bits relating to more important information may beprovided with a higher hamming distance than other data bits.

Preferably, the first station transmits regular signals and wherein thefirst station has a mode and wherein each regular signal asks if thefirst station should change mode, and wherein if the first stationreceives an affirmative response, it changes mode. A mode may comprise apower saving mode in which the regular signals are sent at a slower ratethan the normal mode. This minimises power consumption and is sufficientto allow the second station to maintain synchronisation with the firststation.

Preferably, if the first and second stations are not synchronised, thefirst and second stations will hop between frequency channels atdifferent rates until second station receives the signal andsynchronises with the first station. If the second station detectsbackground noise above a predetermined level on the selected frequencychannel, it will change to a different frequency channel.

A second aspect of the invention provides a transmission system for ameasurement probe for a coordinate positioning apparatus, comprising:

-   -   a first station for mounting with one of the measuring device        and the coordinate positioning apparatus;    -   a second station for mounting with the other of the measuring        device and the coordinate positioning apparatus;    -   wherein the first and second stations may communicate on        different frequency channels and wherein if the second station        hears significant noise on a certain frequency channel, it will        hop to another frequency channel.

Preferred embodiments of the present invention will now be described byway of example with reference to the accompanying drawings wherein:

FIG. 1 illustrates a touch trigger probe mounted on a machine tool;

FIG. 2 is a schematic illustration of the frequency hopping andsynchronisation of the first and second stations;

FIG. 3 is a schematic illustration showing lost hops and eventinterruptions;

FIG. 4 is a schematic illustration showing a probe trigger and delaycounters;

FIG. 5 is a schematic illustration showing synchronisation recovery; and

FIG. 6 is a schematic illustration of the sliding correlator in themachine station.

FIG. 1 illustrates a touch trigger probe 10 mounted on a spindle 12 of amachine tool. The touch trigger probe 10 has a deflectable stylus 14with a workpiece-contacting tip 16. The signal transmission systemcomprises two stations, the probe station 18 is connected to the touchtrigger probe and is mounted to a moving part of the machine tool. Amachine station 20 is mounted on a stationary part 22 of the machinetool structure and is connected to the machine tool controller 24.

Data is transmitted between the probe station 18 and machine station 20using a spread spectrum radio link, in this case a frequency-hoppingradio communications link, which sends discrete packages of serialbinary data.

Both the probe and machine stations hop between different frequencychannels roughly in synchronisation with each other with occasionalmessages sent between them to synchronise the two stations. The probestation initiates each exchange of messages and receives a reply fromthe machine station.

The frequency-hopping and synchronisation will now be described in moredetail with reference to FIG. 2. The machine station is listening formessages most of the time whilst the probe station is in its half-oncondition most of the time (e.g. as in slots n+1 to n+3 above). When theprobe station is half-on its probe interface and microprocessor will beon and the radio modem will be off. The probe interface andmicroprocessor each use about 2 mW of power whilst the radio modem usessignificantly more power, about 120 mW when switched on. The radio modemconsumes a similar amount of power whether it is receiving ortransmitting. The half-on state thus minimises power consumption of thebattery powered probe system.

FIG. 2 shows the probe station turning on with a small settling time andthen transmitting an “I'm OK” message on frequency channel f(n). Theprobe station then listens for the acknowledgement from the machinestation. The machine station which is listening on channel f(n) receivesthis message, synchronises its clock with the probe station and thensends an acknowledgement back on channel f(n). Upon receiving thisacknowledgement the probe station switches back to its half-oncondition. The probe station clock therefore acts as the master clockfor the system. When the machine and probe stations are synchronised,they hop between frequency channels at the same time.

The probe station is now silent for a number of time slots (assumingthere are no probe triggers) and the machine station listens onsuccessive frequency channels f(n+1), f(n+2) etc. Although the probestation is not transmitting on the successive frequency channels f(n+1),f(n+2) etc, it is still hopping between frequency channels. FIG. 2 showsan exaggerated error in clocks between the probe and machine stations.This error is small enough to allow the stations to remain synchronisedto the order of 100 silent time slots. Thus although because of thiserror the machine and probe stations hop to a new channel at a slightlydifferent time, the error is small enough that there is sufficientoverlap when the probe station is transmitting and the machine stationis listening in the same frequency channel for signals to pass betweenthe stations. This error is corrected each time the machine stationreceives a message from the probe station.

For clarity FIG. 2 only shows three silent slots and thus threefrequency-hops are unused. The periodic timer then prompts the probestation to transmit again on f(n+4) and this cycle then repeats untilinterrupted by some other event (e.g. a lost transmission, a probetrigger or a probe station turn-off signal).

Transmissions from the probe station may not be received by the machinestation due to for example interference. Such a situation will now bedescribed with reference to FIG. 3. In FIG. 3 settling time is not shownand the effects of synchronisation of clocks and hopping between theprobe station and machine station is assumed.

The transmitted radio packet from the probe station includes probe data.For example, the probe may be seated (S) or the probe may have triggered(T). Other information may also be transmitted in the radio packet, forexample the condition of the battery, how many transmissions have beenattempted for this message and data relating to the time of a touchtrigger event.

In time slot n a successful message from the probe station and replyfrom the machine station, all on frequency channel f(n) is shown. Thisconfirms that both the probe station and radio link are operating andthat the output from the machine station can be trusted.

In time slot n1 the probe station transmits a message, the machinestation receives this message and sends an acknowledgement. However theprobe station does not receive this reply for example due tointerference.

As no acknowledgement is received the probe station will re-transmit themessage in the next time slot n1+1. FIG. 3 shows the re-transmission ofthe message from the probe station in time slot n1+1. However, as themachine station receives nothing, it does not send an acknowledgement.The probe station will therefore receive no message and so willre-transmit the message in slot n1+2.

In time slot n1+2 everything works. The machine station receives theprobe station message and the probe station receives the machine stationacknowledgement. The probe station can therefore return to its half-oncondition with its radio modem off.

If however after a predetermined time the machine station does notreceive the message from the probe station then either the radio link orthe probe station has failed and the machine station will set an erroroutput.

For the first transmission of a message, a normal radio frequency powerlevel is used, for example 1 mW. On subsequent re-transmissions, theradio frequency power level may be increased, thus increasing the chancethat the message will get through.

As there is the opportunity of re-transmission at a higher radiofrequency power, this enables a slightly lower radio frequency power tobe used for normal transmissions. This has the advantage of minimisingradio traffic and extending battery life.

In time slot n2 in FIG. 3, a probe trigger occurs. An out of sequencetransmission must be sent by the probe station to the machine station assoon as possible. The probe station transmits a probe trigger message tothe machine station in the next time slot n2+1. As before, the machinestation acknowledges the message. A probe trigger message outranks theperiodic update and thus when a probe trigger occurs data relating tothe probe trigger will be included in the data packet sent in the nexttransmission.

As illustrated in FIG. 4, when a probe trigger occurs a timer in theprobe begins counting from zero. The value of this timer t1 is latchedat the beginning of the next time slot n+1. This value t1 is transmittedfrom the probe station to the machine station in a transmission in thenext time slot n+1.

The machine station decodes this value t1 from the transmitted messageand computes a value tk−t1 where tk is a constant. The machine stationloads its own countdown counter with the value tk−t1. At the end of thetime slot n+1 the countdown counter is started and when it reaches zerothe probe status output changes to triggered.

The time delay between the probe trigger and the machine station probeoutput will therefore be t1+ts+tk−t1=ts+tk, where ts is the time of onetime slot. This value ts+tk is constant. The delay between probe triggerand machine station probe output is therefore always the same.

The time constant tk is selected to allow re-transmission of the messageif the first transmission (i.e. in time slot n+1) fails. In FIG. 4, timeslots n+2, n+3 and n+4 are available for re-transmission of the probetrigger message. For a re-transmission a correction is applied to tk−t1equal to the time taken by the number of unsuccessful transmissions.This correction will be the number of retries done multiplied by thelength of a single time slot. The message transmitted by the probestation will include data which indicates which try it is (1st, 2nd,3rd, etc). Alternatively the probe station can (re-)latch the probestation counter at the beginning of each time slot in which a messagewill be sent. (This value will be t1+ts for a message sent in slot n+2,t1+2*ts for a message sent in slot n+3 and so on.) Thus whichever timeslot the message is successfully transmitted in, the total time delaywill be constant (=ts+tk) between the probe trigger and the machinestation probe status output.

For the probe station and machine station to communicate they must bothbe set to the same frequency channel at the same time. To achieve thisthe probe station frequency channel controller and the machine stationfrequency channel controller must be synchronised. This is achieved by asynchronisation recovery/find and collect process described below withreference to FIG. 5.

The probe station is shown hopping between frequency channels at normalspeed (e.g. one hop per millisecond) and the machine station is shownhopping at a much slower speed (e.g. one hop per 50 milliseconds). Theprobe station transmits in every time slot (n,n+1,n+2 etc) and thenlistens for a reply before hopping to the next time slot. The probestation transmission contains the ID number of the probe and includes arequest for synchronisation and acknowledgement of the message. Themachine station listens for many probe station time slots andoccasionally changes to a different frequency channel. In time slotsn,n+1 and n+2 in FIG. 5 the probe station is shown transmitting onsuccessive different frequency channels whilst the machine stationlistens. However whilst the machine station is on a different frequencychannel to the probe station it receives nothing.

In time slot n1−4 the machine station is shown hopping to a newfrequency. Meanwhile the probe station continues hopping frequencychannels and transmitting. In slot n1 the probe station and the machinestation are on the same frequency channel and the machine station hearsthe message from the probe station and synchronises its time slot clockto the probe station. The machine station is now synchronised with theprobe station and a periodic handshake to maintain synchronisation cannow occur. The machine station acknowledges the message from the probestation in the time slot n1.

Usually the acknowledge message from the machine station will bereceived by the probe station. However FIG. 5 illustrates what happensif the probe station fails to hear the acknowledgement. In time slot n1the machine station transmits an acknowledgement but although the probestation is listening it does not receive the acknowledgement. The probestation hops to the next time slot n1+1 and again transmits its message.As the machine station is synchronised it will be listening on thecorrect frequency channel in time slot n1+1 and will thus hear themessage from the probe station. The machine station will synchronise itsclock again and will acknowledge the message again. The probe stationmessage in slot n1+1 is effectively a re-transmission as shown in FIG.3.

During the process of synchronisation recovery, if the machine stationhears significant noise on a certain frequency channel, it willimmediately hop to another frequency channel rather than wait on thefrequency channel where background noise may swamp any transmission fromthe probe station.

It is desirable to be able to turn on the probe station via a radiomessage from the machine station. Whilst waiting for this radio turn-onthe probe station is in its radio standby mode in which it consumessubstantially less battery power than when it is in its operating mode.

The probe station radio standby mode is similar to the periodic update,although the time slots may be wider and the cycle time longer, i.e.slow hopping between frequency channels.

Most of the time the data exchange will consist of the probe stationtransmitting its ID number and asking it if should be turned on, whilstthe machine station replies that it is not needed. As with the operatingmode the machine station is synchronised to the probe station duringthis exchange. If the probe station does not receive an acknowledgementfrom the machine station it will re-try to transmit the message insubsequent time slots in different frequency channels.

If it is required to turn-on the probe station, the machine station willreply “turn-on” and change to operating mode. The probe station willthen switch to the operating mode. In the operating mode the machinestation will maintain synchronisation with the probe station asdescribed above.

Turn-off will require an exchange of messages as the turn-off requestmay come from the machine station or alternatively from the probestation (for example a time out). Following turn-off both probe andmachine stations will return the synchronised slow hopping describedabove.

As discussed earlier, the radio signals between the probe and machinestations consist of message packets of serial binary data. Each messagecontains a header which includes probe station identity data, oraddress, needed to enable the machine station receiver to recognisewhether the message is intended for that receiver and to synchronise aclock in the machine station to the probe station clock.

The machine station uses a correlator to recognise the incoming messageheader.

FIG. 6 illustrates the sliding correlator used in the machine station. Aradio frequency receiver and de-modulator 26 receive radio signalstransmitted from the probe station and output a serial stream ofreceived data into a large shift register 28.

On each pulse of an oversampling clock 30, the incoming serial datastream is sampled and its value (1 or 0) is loaded into the shiftregister 28. Simultaneously the contents of the register are shiftedright 1 bit, the last bit being shifted ‘off the end’ and lost.

A target word is held in a separate target register 32. The entire shiftregister contents are continuously, in parallel, bit to bit comparedwith the target register contents by an array of exclusive-or (EOR)gates 34. One EOR gate is used per bit of the shift register and theoutputs of the EOR gates are added at an Adder 36 to determine thenumber of bit-matches detected.

The number of bit matches detected is then fed to a comparator 38, whereit is compared with the required number of matches threshold 46, whichis typically greater than 95%, to determine the correlation detectedbinary output 42.

The target word is programmable, thus the correlator can be set todetect different desired bit sequences. In particular, the target wordis set to the expected header sequence which will be sent from thetransmitter (i.e. the acquired partner probe station).

In a typical system the header could be a 32 bit word with a data rateof 1 bit/microsecond. The oversampling clock might run at 10 times thedata rate, i.e. 10 MHz and the threshold could be 95% match. Thus theshift register would contain 10×32=320 flip-flops, and the EOR gatearray would contain 320 EOR gates. The outputs of the 320 EOR gateswould be fed to the adder, which would output a number between 0 and 320to the comparator. To achieve a 95% or better match, the threshold wouldbe set at 320×0.95=304 bits. Thus if 304 or more bits in the shiftregister matched their targets from the target word, the correlationdetected output will be True, otherwise it will be False. This test isdone and the correlation detected output updated on every pulse of theoversampling clock, i.e. every 100 nanoseconds.

The advantage of this system is that clock recovery is only required atone end of the half duplex link. The master clock is provided at theprobe station. At the machine station, the sliding correlator is used torecover the clock data from the messages transmitted from the probestation. The sliding correlator provides a reference for the probetrigger time delay and allows acknowledgement messages to be sentalready synchronised, thus removing the need for clock recovery at themaster end of the link (i.e. at the probe station).

There are two main types of possible correlator errors. The correlatormay fail to identify a transmitted message, described above or acorrelator may report a match when no message has been transmitted.

If the machine station falsely believes that it has received a messagefrom the probe station, this will result in loss of the synchronisationof the machine station clock, failure of the radio link and an errormessage being produced. The probe station only listens for a machinestation acknowledgement immediately after it has sent a message and themachine station acknowledgement is thus expected within a very narrowtimeslot.

When in the operating mode, a failure will occur when noise imitates amachine station acknowledgement and thus prevents the probe station fromre-transmitting the message. However the probe station is onlyvulnerable to this error when it is waiting for an acknowledgement whichdoesn't come.

The transmitted message contains several different items of information,such as probe station address, probe status (i.e. seated or triggered),timestamp (i.e. time of probe trigger) and battery status. Some of theseitems have high importance, such as the probe station address and theprobe status. The timestamp has high importance if the probe status is‘triggered’ but is otherwise not important. The battery status has lowimportance.

In order to optimise error protection of the transmission, the mostimportant data bits of the message are encoded with a large hammingdistance.

This allows small numbers of bit errors to be corrected and largernumbers of bit errors to be rejected. A higher hamming distance has theadvantage of allowing some error correction but has the disadvantagethat it increases transmission time. Less important data is providedwith a lower degree of error protection, for example multiple bit errordetection using a cyclic redundancy check.

For example, the probe station address and probe status data may beencoded with a hamming distance of 6, which could allow 1 bit errorcorrection and 4 bit error detection. The timestamp and battery statusmay be encoded with a lower hamming distance of 4 which could provide 3bit error detection.

The information required during the periodic transmissions (probestation address and probe status) thus has higher error protection thanother information in the message. There are several empty timeslotsbetween each periodic transmission which are available forre-transmissions if the transmission fails. However, if all thesetimeslots are used up by unsuccessful re-transmissions, an error signalwill be produced and the whole system will stop. It is thereforeadvantageous to have a high reliability periodic transmission, leavingthe empty timeslots as a safety buffer.

In the event that the probe status is ‘triggered’, the timestamp databecomes important. This data has a lower hamming distance and will beretransmitted if an error is detected.

There may be, for example, about 50 periodic transmissions per secondand about 1 trigger per second. It is therefore more important to avoidre-transmissions on the periodic transmission than the trigger signal.

The system of using longer hamming distance codes for the more importantdata has the advantage that it reduces the number of retransmissionsrequired for the periodic transmissions. As the lower priority data isgiven lower hamming distance codes, the transmission time is reduced.The radio traffic and battery life are thus also reduced.

A feature of the present invention is that once the probe station is inplace, it will only communicate with its partnered machine station. Thisenables different systems to operate simultaneously in the sameenvironment without interfering with one another. A partnering processtransfers the probe stations unique 32 bit ID to the machine station. Ina preferred embodiment of this process, when a probe is put onto amachine, the probe stations enters a ‘Send Acquisition’ mode. In thismode it transmits a message which includes its unique ID and a ‘header’which is recognised by the machine stations. This message is transmittedperiodically, for example once every 1 ms across all channels in itshopping pattern. In a next step, the machine station is powered on andin an initial time period, for example 10 seconds, it is receptive tothe ‘Send Acquisition’ signal sent by the probe station. When themachine station receives the transmission in which it recognises the‘header’, it reads the ID. The machine station saves the ID into itsmemory in the form of an EEPROM (electrically erasable programmable readonly memory) and sends an acknowledgement back to the probe stationwhich contains the same ID. If the probe station successfully receivesan acknowledgement (without errors) containing its own ID, it stops the‘Send Acquisition’ process. The probe and machine station are nowsuccessfully partnered and the machine station will only communicatewith the probe station having this ID.

When the probe and machine stations are partnered (i.e. have the sameID), they will have the same channel hopping pattern and thus will beable to communicate whilst channel hopping.

The probe and machine stations of the transmission systems may transmitsignals using worldwide licence free radio frequency bands. Examples ofsuch bands are 2.4 GHz and 5.6 GHz. An advantage of this is that probeand machine stations may be set to function within these frequency bandsduring manufacture and then the same version of the probe may be usedanywhere in the world.

In the above embodiment, the probe station sends out a regulartransmission to the machine station. However an alternative would be forthe machine station to send out a regular transmission requestinginformation and in response the probe station recording information(i.e. measurement and time information) and transmitting it to themachine station.

The above embodiment describes a regular transmission in which the probeand machine stations synchronise and an out of sequence event driventransmission which contains data about the touch trigger event. However,it is possible to include data relating to the touch trigger event inthe regular sequence of transmissions and thus eliminate the requirementfor out of sequence event driven transmissions. In this case, the datawill need to contain information about the time of the touch triggerevent.

This invention is not limited to touch trigger probes. This transmissionsystem is also suitable for use with scanning probes. In this case theregular transmissions will include data relating to probe deflection andthe time of that probe deflection.

Likewise, the transmission system is suitable for other measurementdevices for use on coordinate positioning apparatus such as machinetools and coordinate measuring machines. A ball bar device is used forperforming a calibration operation on machine tools and coordinatemeasuring machines and is described in U.S. Pat. No. 4,435,905. Thedevice comprises an elongate telescopic bar provided with a ball at eachend. In use, each of the balls is retained in a socket provided on themachine spindle and table respectively and the arm is then driven in acircular path about the centre of the ball retained in the socket on thetable. A single axis transducer provided on the bar measures anyvariation in the centre-to-centre spacing of the balls, and thusdetermines the extent to which the tool holder path varies from acircular path. Data from the single axis transducer is relayed to themachine control via a cable but this has the disadvantage that it limitsthe number of rotations possible with the ballbar. By using thetransmission system of the present invention, the transducer output andcorresponding data may be transmitted in the radio signal, thus removingthe need of the cable and allowing the ballbar to make severalcontinuous rotations. This transmission system may be used on othermeasurement devices, such as temperature probes.

Although the above embodiment describes use of frequency hopping, othertypes of spread spectrum radio link are suitable for use in theinvention, for example direct sequence.

1. A transmission system for a measurement device for a coordinatepositioning apparatus, comprising: a measurement device comprising atouch trigger probe; a first station for mounting with the measurementdevice; and a second station for mounting with the coordinatepositioning apparatus, wherein the first and second stations communicateusing a spread spectrum radio link, the spread spectrum radio link beinga frequency hopping radio link in which the first and second stationsfrequency hop between a series of different frequency channels, in theevent of a measurement event comprising a trigger event, the firststation transmits information relating to the trigger event, includingdata relating to a time of the trigger event, when a signal transmittedby the first station containing information relating to the triggerevent is received by the second station, the second station produces ameasurement device output signal, with a time delay between the triggerevent and the production of the measurement device output signal, whenthe signal transmitted by the first station is not adequately receivedby the second station, the signal is retransmitted by the first station,the time delay being sufficiently long to allow the first station toretransmit the signal within the time delay, the first station comprisesa timer for measuring a time between the trigger event and thetransmission of the signal comprising information relating to thetrigger event, the information relating to the trigger event includingthe time measured by the timer, the retransmitted signal comprisesupdated data relating to the time of the trigger event, and the delaybetween each trigger event and a resulting machine station probe outputis substantially constant.
 2. A transmission system according to claim 1wherein the transmission system uses a worldwide license free radiofrequency band.
 3. A transmission system according to claim 1 whereinthe first and second stations are provided with a clock and wherein theclocks are synchronised at least once.
 4. A transmission systemaccording to claim 3 wherein the first station transmits a regulartransmission and wherein when the second station receives the signal itwill synchronise its clock with the first station.
 5. A transmissionsystem according to claim 4 wherein when the first and second stationsare synchronised, their frequency hopping is synchronised.
 6. Atransmission system according to claim 1 wherein the first stationtransmits a regular transmission and wherein the information relating tothe trigger event is transmitted in an additional transmission.
 7. Atransmission system according to claim 1 wherein, in the event of thefirst station receiving a transmission from the second station, atrigger event is performed by the measurement device and the firststation transmits information relating to said trigger event.
 8. Atransmission system according to claim 1, wherein the first stationtransmits a regular signal and wherein if the second station receivesthe regular signal it will synchronise with the first station; wherein,in the event of the trigger event, the first station transmitsinformation relating to the trigger event in the form of an additionalsignal which includes data relating to a time of the trigger event; andwherein the second station is provided with means for receiving saiddata representing the time and providing a probe output signal derivedtherefrom.
 9. A transmission system according to claim 1 wherein if thesecond station receives the signal transmitted by the first station, ittransmits an acknowledgement signal and if the first station does notreceive an acknowledgement signal in response to its signal, it willre-transmit said signal.
 10. A transmission system according to claim 1wherein the transmission system comprises a half duplex link.
 11. Atransmission system according to claim 1 wherein a master clock isprovided at one end of the transmission system and a sliding correlatoris provided to recover the master clock.
 12. A transmission systemaccording to claim 1 wherein a master clock is provided at one end ofthe transmission system and wherein the master clock provides areference for the measurement device output signal time delay.
 13. Atransmission system according to claim 11 wherein if the second stationreceives a signal from the first station it transmits an acknowledgementsignal and wherein the acknowledgement signal is synchronised with themaster clock.
 14. A transmission system according to claim 1 wherein asignal sent between the first and second stations comprises data bitsand wherein data bits relating to more important information areprovided with greater error protection than other data bits.
 15. Atransmission system according to claim 14 wherein the data bits relatingto more important information may be provided with a higher hammingdistance than other data bits.
 16. A transmission system according toclaim 1 wherein the first station transmits regular signals and whereinthe first station has a mode and wherein each regular signal asks if thefirst station should change mode, and wherein if the first stationreceives an affirmative response, it changes mode.
 17. A transmissionsystem according to claim 1 wherein if the first and second stations arenot synchronised, the first and second stations will hop betweenfrequency channels at different rates until the second station receivesa signal and synchronises with the first station.
 18. A transmissionsystem according to claim 1 wherein if the second station detectsbackground noise above a predetermined level on the selected frequencychannel, it will change to a different frequency channel.
 19. Atransmission system according to claim 1 wherein the first station hasan ID code and wherein the second station can be set to only communicatewith the said first station having said ID code.
 20. A transmissionsystem according to claim 1 wherein the first station is provided with amode in which it transmits a signal containing its ID code and thesecond station is provided with a mode in which on receiving saidsignal, it is set to only communicate with the first station having thisID code.